I think you're going a little far with this. Last season was his best season imo, and he really isn't much worse this season than in either of the previous two... If anything I'd say what's disappointing is that he's plateaued, not that he's regressed. I just don't see the regression.
He sees more of the ball this year than in either of the past two seasons. And has more responsibilities... But his tackling, interception, and clearance stats have been within a narrow margin for 3 years... His passing numbers, passing accuracy, key passes per game, long balls per game are identical the last 3 years... His successful dribbles and fouls won have been identical... and he's not being dispossessed or making bad touches more frequently than the last 2 years. His numbers went slightly up from 13/14 to 14/15, and have stayed the same this year 15/16.
He's also won possession in the final third of the pitch more times than any player in Serie A this season (25 times going into the game against Atalanta)... which is very impressive.
Where you see that he's improved leaps and bounds over the past 2 seasons is in the CL. More passes, more key passes, more long balls, more successful dribbles (by a large amount), dispossessed less, more shots, more tackles, more interceptions... He's imposing himself upon games in Europe far more than the last 2 years.
Serie A, he's levelled off, and needs to get his head straightened out and show he's motivated to continue becoming a world class player.